1. Field of the Invention
The present invention generally relates to a sequence instruction display system for use with a programmable sequence controller for displaying a sequence instruction and the on-off state of a corresponding input or output element.
2. Description of the Prior Art
A programmable sequence controller automatically controls a controlled-objective such as a machine tool in accordance with a sequence program, which includes a large number of sequence instructions. The prior art display system is capable of displaying desired sequence blocks, each of which is made up of one output element and several input elements, and further displaying the on-off state of the elements. Such a system is disclosed in U.S. Pat. No. 3,964,026 to Nobuharu Yamauchi, et al. Using the prior art display system, an operator can check a sequence block when a machine problem has occurred or, in other words, when an output element has not been energized, and he can determine the cause of the machine problem or an input element which concerns the energization of the output elements. Further, the operator can check sequence instructions of a certain sequence block and modify the same if desired.
However, such a prior art display system can be used only with a sequence controller which has the function of accepting an interrupt signal from the display system so as to halt the execution of the sequence control operations thereof and to send the sequence program stored in a program memory thereof to the display system. The prior art display system is provided with means to designate a memory address of the program memory so that a certain sequence instruction is transferred to the display system during the period when the execution of the sequence control operations is being interrupted. The interruption may cause a problem in the sequence control operations, because the scan of the program memory in the sequence controller is interrupted thereby to affect controlling a controlled objective.
Further, some sequence controllers, which are relatively small and inexpensive, have no such interruption function. In particular, a so-called one board sequence controller, which is made of plated printed circuits on a single substrate, does not have such a function. Therefore, it is desired to provide a display device which can be used with a sequence controller with no interruption function, and it is also desired to provide a display system which does not interrupt the execution of the sequence control operations of a sequence controller.